The first man-made plastic was invented in Britain in 1851 by Alexander Parkes. Mr. Parkes publicly demonstrated his invention at the 1862 International Exhibition in London, calling his invention Parkesine. Parkesine is derived from cellulose and can be heated and molded. The parkesine retained its shape after it was cooled. It was, however, expensive to produce, prone to cracking, and highly flammable. In 1868, American inventor John Wesley Hyatt developed a plastic material he named Celluloid, which was an improvement of Parkes' invention. Hyatt patented the first injection molding machine in 1872. It worked like a large hypodermic needle, using a plunger to inject plastic through a heated cylinder into a mold. The industry expanded rapidly in the 1940s because World War II created a huge demand for inexpensive, mass-produced products. In 1946, American inventor James Watson Hendry built the first screw injection machine. This machine also allowed material to be mixed before injection, so that colored or recycled plastic could be added to virgin material and mixed thoroughly before being injected. In the 1970s, Hendry went on to develop the first gas-assisted injection molding process.
Injection molding machines consist of a material hopper, an injection ram or screw-type plunger, and a heating unit. These machines also have presses that hold the molds in which the components are shaped. Presses are rated by tonnage, which expresses the amount of clamping force that the machine can exert. This force keeps the mold closed during the injection process. Tonnage can vary from less than five tons to 6,000 tons, with the higher figures used in comparatively few manufacturing operations. The total clamp force needed is determined by the projected area of the part being molded. This projected area is multiplied by a clamp force of from two to eight tons for each square inch of the projected areas. As a rule of thumb, four or five tons per square inch can be used for most products. If the plastic material is very stiff, it will require more injection pressure to fill the mold, thus more clamp tonnage to hold the mold closed. The required force can also be determined by the material used and the size of the part, larger parts require higher clamping force. With injection molding, granular plastic is fed by gravity from a hopper into a heated barrel. As the granules are slowly moved forward by a screw-type plunger, the plastic starts to melt and is forced into a heated chamber, where it is completely melted. As the plunger advances, the melted plastic is forced through a nozzle that rests against the mold, allowing it to enter the mold cavity through a gate and runner system. The mold remains cold so the plastic solidifies almost as soon as the mold is filled.
Mold assembly or die are terms used to describe the tooling used to produce plastic parts in molding. The mold assembly is used in mass production where thousands of parts are produced. Molds are typically constructed from hardened steel, etc. Mold assemblies are used in molding systems, along with mold assemblies, for the manufacture of plastic articles. Usually, hot-runners systems and mold assemblies are treated as tools that may be sold and supplied separately from injection molding machines and presses.
United States Patent Publication Number 2009/0274790 (Inventor: JENKO, et al.; Filed: 17 Jul. 2009) discloses a hot runner system including a shooting pot system for transferring melt from a single shooting pot to multiple nozzles. Melt is fed from a source of melt into the cavity through the multiple nozzles, and a valve isolates melt in the cavity from melt in the source. A plunger within the cavity is driven forward to inject melt in the cavity into a mold cavity at high pressure without significantly increasing the pressure of melt in the source. The plunger optionally functions as both the plunger and the valve by opening and closing communication between the cavity and the manifold as it is rotated.